It also found a 6.12ct type II blue stone at the mine.
Petra said the recoveries of the two diamonds demonstrated "the prevalence of these types of stones in the Cullinan orebody, as well as the ability of the mine's plant to recover the full spectrum of diamonds".
It will be selling both stones in its upcoming March tender.
At 20.94p (US27.61c), Petra's shares have fallen 38% in the past six months.